Markdown subscript migration

[liamwhite]

This changes the subscript operator to the Pandoc style single tildes (~), and adds a release task to automatically reformat all subscript operators encountered in Markdown text.

Deployment instructions:
First, deploy as usual. Then, select one:

1. Migrate all tables automatically:
   philomena eval "Philomena.Release.migrate_markdown(:all, -1, -1)"
2. Migrate each table individually or in parallel, with start and end ids:

   philomena eval "Philomena.Release.migrate_markdown(:images, 0, 100000)" &
   philomena eval "Philomena.Release.migrate_markdown(:images, 100000)", 200000)" &
   # ...
   

Reindexing images, comments, posts, reports and tags is also recommended but not required, since Markdown syntax is ignored by the search stemming engine.